1 64 THE LOSS OF WATER FROM PLANTS 



conditions the rates of these two processes are virtually equal, this is not always 

 true, particularly if an internal water deficit exists in the plant. The rate of 

 transpiration as measured for a cut shoot in a potometer does not necessarily 

 bear any relation to its rate of transpiration while it was still attached to a 

 plant. The reasons for this will become clear in the discussion of the internal 

 water relations of plants (Chap. XVIII). The principal utility of potom- 

 eters is in laboratory demonstrations of the effects of various environmental 

 factors upon the rate of transpiration. 



4. Hygrometric Paper Methods. — ^When filter paper is impregnated with 

 a dilute (about 3 per cent) solution of cobalt chloride and dried it becomes 

 a bright blue in color. If exposed to moist air its color gradually changes to 

 pink. The same color change ensues when a piece of such paper is brought 

 into contact with the transpiring surface of a leaf. If small pieces of such 

 paper are so mounted as to be protected from the water-vapor of the atmos- 

 phere by glass, mica, or celluloid, and are brought into contact with the sur- 

 face of a leaf the rate at which the paper changes in color from blue to pink 

 is an indication of the rate at which water-vapor is being lost by that leaf. 

 A leaf which changes the color of a piece of cobalt chloride from its full blue 

 color to its full pink color in, for example, 30 seconds is losing water to the 

 paper twice as fast as a leaf which requires 60 seconds to accomplish a similar 

 change. This method gives no measure of absolute rates of transpiration, 

 because when a portion of a leaf is covered with a piece of cobalt chloride 

 paper the environmental conditions influencing the leaf under the paper are 

 very different from those which would influence it were it freely exposed to 

 the atmosphere. The leaf under the paper is exposed to a reduced light in- 

 tensity and an initially lower vapor pressure than freely exposed parts of the 

 same leaf, and furthermore, is completely isolated from any effects of wind. 

 Hence the rate of loss of water to the paper may be very different from the 

 rate of water loss of the same area of leaf surface to the atmosphere. Under 

 certain conditions this method can be used for a determination of the relative 

 rates of transpiration of different species with a fair degree of accuracy. Even 

 for relative determinations of transpiration rates, however, this method gives 

 valid results only when all of the plants are growing under essentially the same 

 atmospheric conditions. Modifications and limitations of this method are 

 discussed by Livingston and Shreve (1916) and Meyer (1927). 



The transpiration rates of plants may be expressed in terms of a unit of 

 leaf surface (usually per square decimeter), per unit of fresh or dry weight 

 of the leaves or tops of the plant, or per plant. Each of these modes of ex- 

 pression of transpiration rates is of value for certain purposes. 



